The present invention relates, in general, to a magnetic recording drive, utilizing magnetic recording, such as a magnetic disc drive, a magnetic tape drive, and a magnetic floppy disc drive.
A magnetoresistive (MR) sensor operates to detect a leakage magnetic field from a recorded magnetic transitions by utilizing the fact that the resistivity of the sensor depends on the leakage magnetic field. Therefore, when the magnitude of the sense current which flows through the sensor is increased, the large reproduced output signal is obtained irrespective of the relative speed between the head and the medium such as disc and tape. For this reason, the magnetoresistive sensor has been widely used in the reproducing heads of the high density magnetic recording drives. However, in order to obtain the large output, the magnitude of the current (the sensor current), which flows through the magnetoresistive (MR) sensor, needs to be increased. On the other hand, since the sense current flows through a very thin MR sensor layer, it is necessary to prevent disconnection due to electromigration and the like. For this purpose, heretofore, only during the operation for reproducing the recorded signals, the sense current was caused to flow through the MR sensor.
Along with the promotion of high density in the magnetic disc drive, it is desired to improve the head sensitivity to secure the read operation of the reproduced output. Thus, if a large reproduced output is intended to be obtained by increasing the sense current which flows through the MR sensor, since in general, the current density of the sense current is so large as to be 10.sup.7 A/cm.sup.2 or so, in terms of the useful life of the sensor, there arises the problem that the sensor is degraded due to the electromigration or the acceleration of a electromigration due to the temperature during operation.
In addition, for the purpose of increasing the track density, in order to improve the head positioning accuracy, the embedded sector servo utilizing the positioning signal on the data surface has been widely used. Then, if in this method, the magnetoresistive (MR) sensor is used for reproducing both the recorded read-write (R-W) data and the positioning signal, the following problem will arise. In the embedded sector servo method including the reproducing operation, the positioning signal is always reproduced which is recorded in each portion which is called the sector and is concentrically distributed in a fan shape on the magnetic disc. Therefore, after the reproducing of R/W data has been performed by using a certain head, a following operation is continued while the positioning signal is reproduced by the same head until the next instruction of R/W is issued. Therefore, in the case of a head which has to read out the information having the high reproducing frequency such as a directory, the total reproducing time of the MR head including the time required for the positioning signal is remarkably increased as compared with other heads.
For example, the case of the magnetic disc drive having the following specifications will hereinbelow be discussed.
(1) Number of sectors: 82 sectors/track Positioning signal: 99 Bytes/sensor (including ID portions) Frequency of occurrence of recording/reproducing operation: 5 times/sec Amount of look ahead cache data: 2 tracks Number of heads: 15 heads Rotation of frequency of the disc: 4,500 rpm PA1 (2) Useful life: 500,000 hours Operating time; 12 hours/day Ratio of recording to reproducing: 1/3 PA1 (3) Total R/W data reproducing time: 4,990 hours PA1 (4) Total positioning signal reproducing time: 3,800 hours
The operating conditions of the above-mentioned disc drive are set as follows.
FIG. 1 shows the data format of each track. As can be seen from the figure, each track includes, but is not limited to, a track ID portion and a plurality of sector portions which follow an INDEX representing a starting point of the track. In each sector, the positioning signal is recorded in the head portion thereof, and next both an ID portion and a data portion of the sector are recorded. Therefore, the selected head always continues to reproduce the positioning signal for the following operation. On the other hand, in the recording and reproducing modes, the ID portion is also read out, and the objective sector is found out. After the ID portion has coincided with the objective value, in the recording mode, the recording of the data is performed. In the reproducing mode, on the heels of the reproducing of the sector to be reproduced, in general, the look ahead for one or two tracks is performed.
Therefore, the following results are obtained.
As described above, for a period of time ranging from a time point when the recording or reproducing operation has been temporarily completed up to a time point when the subsequent recording or reproducing is performed, the same head HDn is selected. For that period of time, the positioning signal in the following operation will be also reproduced with the same head HDn. As the excessive case, if it is assumed that the processings are concentrated on one head, it takes in total 8,790 hours, i.e., 366 days. Thus, it is very difficult to ensure the useful life of the magnetoresistive sensor type head in the above-mentioned high current density state.